Quasi-CW UV laser with low peak pulse-power

Abstract

Laser apparatus is disclosed in which fundamental-wavelength optical pulses delivered from a mode-locked laser resonator at a pulse-repetition frequency (PRF) are converted to harmonic-wavelength pulses in an optical delay loop. One example is disclosed in which the harmonic-wavelength pulses are delivered directly from the delay loop. Another example is disclosed in which the harmonic-wavelength pulses are divided by the delay loop into a number of temporally spaced-apart replicas thereof, and the delay loop delivers bursts of replicas of different one of the harmonic wavelength pulses at a burst-repetition frequency equal to or a multiple of the PRF of the resonator.

1. Laser apparatus comprising:
 a laser-resonator arranged to deliver fundamental-wavelength optical pulses having a pulse-duration, the fundamental-wavelength optical pulses being temporally equispaced by a pulse-repetition period τ and delivered at a pulse-repetition frequency (PRF) equal to 1/τ; and 
 an optical delay loop including one or more optically nonlinear crystals and arranged to receive the fundamental-wavelength optical pulses, the optical delay loop being configured and arranged such that each fundamental-wavelength pulse is converted within the optical delay loop into a harmonic-wavelength pulse and further arranged to convert each of the harmonic-wavelength pulses into a plurality of replicas thereof, with the replicas of each harmonic-wavelength pulse being temporally spaced apart by τ/N±Δτ, where N is an integer equal to or greater than 2 and Δτ is about equal to or greater than the pulse-duration of fundamental-wavelength optical pulses, the optical delay loop being further arranged to deliver bursts of the harmonic-wavelength pulse-replicas at a burst-repetition frequency equal to the PRF of the fundamental-wavelength optical pulses multiplied by N, with each burst including replicas of different ones of the harmonic-wavelength optical pulses. 
 
     
     
       2. The apparatus of  claim 1 , wherein the optical delay loop is configured such that the replicas in each burst thereof are delivered along a common path. 
     
     
       3. The apparatus of  claim 1 , wherein the delay loop is configured such that the number of replicas in each burst thereof is predetermined and is further configured such that each of the replicas in a burst thereof is delivered along separate path, with the separate replica-paths being spaced apart and parallel to each other, and such that each of the replicas in a burst has about the same intensity. 
     
     
       4. Laser apparatus comprising:
 a laser-resonator arranged to deliver fundamental-wavelength optical pulses having a pulse-duration, the fundamental-wavelength optical pulses being temporally equispaced by a pulse-repetition period τ and delivered at a pulse-repetition frequency (PRF) equal to 1/τ; and 
 an optical delay loop including two or more optically nonlinear crystals and arranged to receive the fundamental-wavelength optical pulses, the optical delay loop and the optically nonlinear crystals therein being configured and arranged such that each fundamental-wavelength pulse is converted within the optical delay loop by the optically nonlinear crystals into a plurality N of harmonic-wavelength pulses, with the harmonic wavelength pulses being delivered from the delay loop along a common path, temporally spaced apart such the PRF of the harmonic-wavelength pulses on the common path is N times the PRF of the fundamental wavelength pulses. 
 
     
     
       5. The apparatus of  claim 4 , wherein the optical delay loop includes four optically nonlinear crystals arranged in first and second pairs thereof, with the first pair of crystals generating a first third-harmonic-wavelength pulse from a fundamental-wavelength input pulse, and the second pair of crystals generating a second third-harmonic-wavelength pulse from a portion of the input fundamental-wavelength pulse residual from the generation of the first third-harmonic-wavelength pulse, whereby the PRF of third-harmonic-wavelength pulses in the common path is twice the PRF of the fundamental-wavelength pulses. 
     
     
       6. The apparatus of  claim 5 , wherein the third-harmonic-generation efficiencies of the pairs of crystals are adjusted relative to each other such that the first and second third-harmonic-wavelength pulses generated from a fundamental wavelength input pulse have equal intensity. 
     
     
       7. Laser apparatus comprising:
 a laser-resonator arranged to deliver fundamental-wavelength optical pulses having a pulse-duration, the fundamental-wavelength optical pulses being temporally equispaced by a pulse-repetition period τ and delivered at a pulse-repetition frequency (PRF) equal to 1/τ; 
 a first harmonic-generator arranged to generate a first harmonic-wavelength pulse from each one of the fundamental-wavelength pulses; 
 a second harmonic-generator arranged to generate a second harmonic-wavelength pulse from a residual portion of the fundamental-wavelength pulse; and 
 an optically dispersive device located between the first and second harmonic-generators and configured to temporally separate the first harmonic-wavelength pulse and the residual portion of the fundamental-wavelength pulse by a predetermined period, such that the first and second harmonic-wavelength pulses are temporally separated by about the same predetermined period. 
 
     
     
       8. The apparatus of  claim 7 , wherein the first and second harmonic-wavelength pulses are temporally separated by about one pulse-duration of the fundamental-wavelength pulses. 
     
     
       9. A method of generating harmonic-wavelength pulses from fundamental-wavelength pulses delivered from a mode-locked laser-resonator the fundamental wavelength pulses having a pulse-duration, the fundamental-wavelength optical pulses being temporally equispaced by a pulse-repetition period τ and delivered at a pulse-repetition frequency (PRF) equal to 1/τ, the method comprising the steps of:
 optically dividing each fundamental-wavelength pulse into a plurality N of portions propagating along N different optical paths; 
 converting the fundamental wavelength pulse-portions in each of the N different optical paths into a M th -harmonic-wavelength pulse, such that N M th -harmonic-wavelength pulses are generated from each fundamental wavelength pulse; and 
 combining the separate paths at a common location, with the different optical path lengths selected such that the M th -harmonic-wavelength pulses at the common location are temporally spaced apart from one from another by at least Δτ, where Δτis about equal to or greater than a pulse-duration of the fundamental-wavelength pulses. 
 
     
     
       10. The method of  claim 9 , wherein the different optic path lengths are selected such that temporally sequential ones of the M th -harmonic-wavelength pulses are temporally spaced apart by a period spaced apart by τ/N±Δτ. 
     
     
       11. The method of  claim 10 , wherein N is 2, and the M th -harmonic-wavelength pulses are third-harmonic-wavelength pulses. 
     
     
       12. The method of  claim 11 , wherein the M th -harmonic-wavelength pulses are combined on a common path.